1. Field of the Invention
This invention relates to a method for forming a barrier layer in a contact, and particularly relates to a method for forming a barrier layer in a contact of high aspect ratio.
2. Description of the Prior Art
To increase the ohmic contact between the barrier layer and the silicon containing layer, a Ti layer is frequently deposited between the silicon containing layer and the TiN layer. The TiN layer is used as a barrier layer between a metal layer and a silicon containing layer. When forming a plug in a contact, the barrier layer is usually used to prevent the spike produced at the interface of the plug (usually formed of metal tungsten or aluminum) and the silicon containing layer. The TiN (titanium nitride) is usually used to form the barrier layer mentioned above. So the Ti layer together with the TiN layer is frequently used before the deposition of plug or interconnection.
To form the TiN barrier layer and the Ti layer in a contact, the ionized PVD (Physical Vapor Deposition) or the traditional collimator is frequently used in the semiconductor industry. Though the step coverage of the TiN layer and the Ti layer is improved, yet the step coverage of the TiN layer and the Ti layer in a contact of high aspect ratio is still a problem.
Take a tungsten plug fabricating process for example, before forming the tungsten plug, referring to FIG. 1, it is necessary to form a contact in the inter poly oxide layer 10 on the silicon contained layer 11, the inter poly oxide layer 10 is made of BPSG (Borophosphosilicate glass). Using the developed photoresist layer 13 as a mask, the etching stops at the surface of the silicon contained layer 11. The next step is to strip the photoresist layer 13, and to remove the residue polymer after forming the contact. The cross sectional view of the semiconductor wafer is shown in FIG. 2, in which the photoresist layer is removed, and a Ti layer 15 and a TiN layer 17 are subsequently formed on the topography of the semiconductor wafer. The silicon contained layer 11 can be a silicon substrate.
The method used to form the Ti layer and the TiN layer can be an ionized PVD (Physical Vapor Deposition) or a traditional sputtering technology using collimator. As shown in FIG. 2, because the contact is of high aspect ratio, and the method to form the barrier layer is the traditional method mentioned above, the step coverage of the Ti layer 15 and the TiN layer 17 is very poor. To form the tungsten plug, a portion of the Ti layer 15 and the TiN layer 17 on the inter poly oxide layer 10 is removed. As shown in FIG. 3, a first portion of the silicon contained layer 11 is not covered by the Ti layer 15 and the TiN layer 17 at the bottom of the contact. Whereas a second portion of the silicon contained layer 11 is covered by the Ti layer 15 and the TiN layer 17 at the bottom of the contact. To form the tungsten plug in the contact, the WF.sub.6 is induced into the reaction chamber, as shown in FIG. 3. The WF.sub.6 penetrates the first portion of the silicon contained layer 11 in the contact to react with the exposed Si and the exposed Ti, so the worm hole 19 in the silicon contained layer 11 is thus produced. The problem mentioned above becomes very serious especially when the contact is of a bowing profile.
Because of the worm hole 19, the Si leakage become very serious, and the electric characteristic of the device using the plug becomes worse. Referring to FIG. 4, a tungsten plug 21 is formed in the contact. Because it is very difficult to form the PVD barrier layer at the bottom of the contact with good step coverage, the tungsten plug formed in the forgoing steps may have high resistivity because of the worm hole. Especially, in the high aspect ratio contact, the condition mentioned above becomes more serious. To improve the problems mentioned above, it is necessary to improve the step coverage of the TiN layer 17 to prevent the WF.sub.6 from reacting with the Si in the silicon contained layer 11 and the Ti in the Ti layer 15. Thus prevent the worm hole from being produced.